An old Mac will boot automatically from any floppy disk that is in the drive at startup. So, to boot from a floppy, with the Mac off:
1. Stick the floppy partway into the drive
2. Reach around and flip the switch on the back, and immediately
3. Push the floppy the rest of the way into the drive.

If it spits the floppy back out again, it's not bootable.